Performance Testing of Echo Cancellers Using a White Noise Test Signal

ABSTRACT

Testing equipment for and a method of performance testing an echo canceller are disclosed. The method includes generating a white noise test signal that includes a plurality of high-level white noise time segments, each of which is separated by a low-level white noise time gap. The generated white noise test signal is transmitted to the echo canceller. At least one echo output signal is received from the echo canceller, and the echo output signal results from the white noise test signal. The level and duration of the received at least one echo output signal are measured.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/740,102 filed Dec. 17, 2003, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to communication systems and,more particularly, to a method for the objective performance testing ofan echo canceller using a segmented test signal and echo measurementalgorithm.

BACKGROUND OF THE INVENTION

All voice network service providers who want to introduce packettransport technology or any other technology (e.g., digital cellular)that increases connection delay beyond approximately 25 msec (roundtrip) need to deploy echo cancellers within their networks and thesedevices need to perform their function across a wide range of usagevariation. The basic objective of a well-performing echo canceller (EC)is to quickly recognize an echo and then rapidly “converge” to cancelthe echo by reducing it in level below the threshold of notice.Accordingly, it is very important to test an echo canceller (EC) priorto deployment in the network to ensure that it will perform toexpectations.

A challenge in the area of echo canceller (EC) performance testingconcerns what test signal(s) to use. Today, the standard EC test isbased on International Telecommunication Union-Standardization Sector(ITU-T) Recommendations G.165 and G.168. ITU-T Recommendation G.165recommended the use of band-limited (300-3400 Hz) white noise as thetest signal. The updated, G.168 Recommendation defined and recommended anew test signal, the composite source signal (CSS) and the pass/failcriteria is based on results generated with this signal.

The CSS is a speech like signal in that it has a power density spectrumsimilar to that of speech, it is interrupted by gaps that simulate thepauses found in speech, and it simulates both voiced and unvoicedsounds. Like all good test signals, the CSS can also be specificallydefined and, thus, constructed with fidelity in the different test labs.The CSS is generally regarded as a superior test signal to the whitenoise test signal that it replaces and performance seen with the CSS isassumed to be predictive of how the EC will perform in the presence ofspeech. However, there is ample evidence that shows that this predictiontoo often does not hold.

The existing “artificial” test signals used in the telecommunicationsindustry for evaluating the performance of an echo canceller (EC) (i.e.,those recommended by the ITU-T in Recommendations G.165 and G.168)frequently fail to predict how the echo canceller will perform in “real”uses (i.e., when the EC is working on actual speech signals).Specifically, testing an echo canceller with a CSS signal is not alwayspredictive of its performance in the presence of speech. Consequently,pre-deployment performance testing of new EC designs does not providethe level of confidence desired in that the performance seen in the labis not always the performance experienced in the field.

Since echo cancellers are deployed to control the echo of speechsignals, the test signal of choice would seem to be a speech signal(s).It has been proposed to use a speech signal as the test signal, butthere has been no agreement in the industry as to which speech signal touse. Speech signals vary widely from person to person and EC performanceis sensitive to this variation.

Attempts to substitute speech signals in the G.168 testing revealsubstantial performance variation driven by the specific speech samplein use. So it would appear that a sample of speech signals needs to beidentified and used. This conclusion is not a satisfying one. G.168 ECtesting is complicated enough with a single test signal in use. Tryingto get general agreement to move to a sample of speech signals fortesting is likely to be difficult if not impossible. Furthermore, to getlab-to-lab conformity, these signals would have to be shared since theirindependent reproduction is also not likely due to the complexity ofreal speech signals.

One approach to handling this complexity is to use multiple speechsignals in an EC performance test and to score EC performance in termsof the results of a subjective mean-opinion-sore (MOS) test, wheregroups of test subjects listen to the processed speech samples and ratethem in terms of their transmission quality based on the residual echoseen with the speech samples. The problem with this approach is that itdoes not provide the single, objective test signal needed to do routinetesting and for setting objective performance requirements. Also, theMOS technique is rather expensive to implement and, under idealconditions, can take days to conduct.

Accordingly, there is a need for an improved “objective” test signal foruse in echo canceller performance evaluations. The currently usedsignals of band limited white noise and the CSS both have generalutility but neither represent speech as well as they should. Thus, itwould be desirable to provide an artificial test signal which generatesperformance that correlates highly with that observed with actual speechsignals.

SUMMARY OF THE INVENTION

The present invention is a method for the performance testing of an echocanceller. The method generally includes generating a white noise testsignal that includes a plurality of high-level white noise time segmentseach of which is separated by a low-level white noise time gap,transmitting the test signal to the echo canceller, receiving at leastone echo output signal from the echo canceller as a result of the whitenoise test signal, and measuring level and duration of the received atleast one echo output signal. A performance score can be calculated forthe echo canceller based on the measured level and duration of the atleast one echo output signal.

In a preferred embodiment, the performance score is calculated bymultiplying the measured level and duration of the at least one echooutput signal and a pass/fail criterion can be developed based on thecalculated performance score as compared to mean opinion score resultsof multiple speech sample testing. Also, the echo output signalpreferably has a duration greater than about 5 msec and a level greaterthan about −50 dBm. Preferably, the performance score is calculated bysumming the products of the measured level multiplied by the durationfor each of echo output signal.

The test signal of the present invention includes time segments of whitenoise (white noise segments) that are associated with frequencysub-bands (bands) of an overall frequency band ranging from about 0 kHzto about 3.5 kHz. In a preferred embodiment, the test signal has fourwhite noise segments, wherein each white noise segment of the testsignal is repeated in succession, whereby the test signal has a total ofeight white noise segments. Preferably, the test signal has a firstwhite noise segment associated with a first frequency band of about 0kHz to about 1 kHz, a second white noise segment associated with asecond frequency band of about 1 kHz to about 2 kHz, a third white noisesegment associated with a third frequency band of about 2 kHz to about 3kHz and a fourth white noise segment associated with a fourth frequencyband of about 3 kHz to about 3.5 kHz.

The test signal of the present invention further preferably has aplurality of white noise segments which are ordered within the testsignal by increasing frequency. The plurality white noise segments alsopreferably have an equal level of about −18 dBm and an equal duration ofabout 350 msec. The plurality of white noise segments of the test signalare further preferably separated from one another by a white noise timedelay or gap of about 150 msec. The level of the white noise gap isabout −65 dBm.

Other objects and features of the present invention will become apparentfrom the following detailed description considered in conjunction withthe accompanying drawings. It is to be understood, however, that thedrawings are designed as an illustration only and not as a definition ofthe limits of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of the segmented test signal formedin accordance with the present invention.

FIGS. 2 and 3 show some convergence speed results for two different echocancellers in response to the 4-band signal formed in accordance withthe present invention.

FIG. 4 shows the MOS result for four ECs tested with the test signal ofthe present invention at 5 ERL levels.

FIG. 5 plots the tested relationship between the Erep calculated on therecordings made with the speech signals and the MOS results across thefour ECs of FIG. 4.

FIG. 6 plots the tested Erep for the 4-band signal as a function of bothEC and ERL for the four ECs of FIGS. 4 and 5.

FIGS. 7 a, 7 b and 7 c plot MOS against the tested Erep for each of the4-band, white noise and CSS signals.

FIG. 8 shows the correlation between the 4-band test signal results andthe speech samples' Erep score.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Frequency variation is a major source of speech sample variations thatare actually observed, especially across speaker gender and age. It hasbeen found that to address the need for a more representative objectivetest signal, a test signal that combines the attributes of beingobjective (i.e., being definable at a level that allows others torecreate it with fidelity), and being representative of speech signals(i.e., that it generates convergence speed performance results thatcorrelate very well with MOS results obtained with actual speechsignals) is most desired.

FIG. 1 shows the new test signal 10, for the objective performancetesting of an echo canceller, formed in accordance with the presentinvention. The test signal 10 is a segmented white noise signalincluding four (4) separate time segments 12 a, 12 b, 12 c and 12 d ofwhite noise (or white noise segments). The white noise segments 12 a, 12b, 12 c and 12 d are associated with respective frequency sub-bands (orbands) of an overall frequency band ranging from about 0 kHz to about3.5 kHz. Preferably, each white noise segment 12 a, 12 b, 12 c and 12 dappears twice in succession (or is paired), thus resulting in the signal10 having eight (8) white noise segments 12 a, 12 a′, 12 b and 12 b′, 12c, 12 c′, 12 d and 12 d′ as shown in FIG. 1. The four white noisesegments 12 a, 12 b, 12 c and 12 d (including paired white noisesegments 12 a′, 12 b′, 12 c′ and 12 d′) are respectively associated withdifferent frequency bands: 0-1 kHz 13 a, 1-2 kHz 13 b, 2-3 kHz 13 c, and3-3.5 kHz 13 d. In the overall test signal 10, these white noisesegments are ordered in terms of increasing frequency as shown in FIG.1, such that the first white noise segment pair 12 a, 12 a′ isassociated with a first frequency band 13 a of 0-1 kHz, the second whitenoise segment pair 12 b, 12 b′ is associated with a second frequencyband 13 b of 1-2 kHz, the third white noise segment pair 12 c, 12 c′ isassociated with a third frequency band 13 c of 2-3 kHz and the fourthwhite noise segment pair 12 d, 12 d′ is associated with a fourthfrequency band 13 d of 3-3.5 kHz. All white noise segments arepreferably at a high-level of about −18 dBm and preferably have aduration of about 350 msec. Also, the white noise segments arepreferably separated from one another by a low-level white noise delayor gap 14 having a duration of about 150 msec. It has been found thatthe high-level white noise segments approximate average speech over thepublic switched telephone network (PSTN) and the low-level white noisegaps 14 approximate pauses in speech patterns. Additionally, the whitenoise floor of the test signal (or the white noise gaps 14) ispreferably at a low-level of about −65 dBm. For short-hand convenience,the test signal 10 described above is also termed as a 4-band signalbelow.

FIGS. 2 and 3 show some convergence speed results for two different echocancellers (ECs) in response to the 4-band signal 10 formed inaccordance with the present invention. The segmented test signal 10passing though the EC in the send direction FI (G.168 term S_(in)) andNO (G.168 term R_(out)) EC ports is shown, as is the echo spikes leakingthrough the EC in the return direction FO port (G.168 term S_(out)). Forthe EC whose performance is shown in FIG. 2, there is no apparentinitial echo at the lowest frequency band (0-1 kHz) of white noisesegment 12 a. In both the second band (1-2 kHz) of white noise segment12 b and the third frequency band (2-3 kHz) of white noise segment 12 c,a brief spike of echo is detected, but full convergence (no echo) isdetected in the white noise segments 12 b′ and 12 c′. In the fourthfrequency band (3-3.5 kHz) of white noise segment 12 d, a brief spike ofecho is detected, and an even briefer, and attenuated, spike is detectedin the white noise segment 12 d′. The pre-convergence detected in thelowest frequency band of white noise segment 12 a is possibly a resultof that EC declaring that low frequency as noise.

The second EC shows a different convergence pattern as illustrated inFIG. 3. At the beginning of the test signal 10, initial echo is detectedin the lowest frequency band of white noise segment 12 a that iscaptured (converged on) during white noise segment 12 a′. This samepattern is detected in the frequency band (1-2 kHz) of white noisesegment 12 b and white noise segment 12 b′. Basically, no echo isdetected in the next frequency bands of white noise segments 12 c and 12d. Given the frequency-time confounding inherent in the test signal 10,it is unclear whether the particular design of this EC allows adaptivefilter control gained at lower frequencies to cause the NLP to functionimmediately for the higher frequency energy or whether this EC ispre-converged for high frequency energy.

Nevertheless, these results show that the test signal 10 according tothe present invention reveals convergence performance both within andbetween frequency bands. This is the type of sensitivity that is mostdesired in an echo canceller test signal.

To expand the utility of the new test signal 10, a new method fortesting the performance of an echo canceller is also provided. The firstrequirement of any useful EC convergence performance measure is that itlink to the customers' experience. The psychophysics of human hearingindicates that, in sound detection, energy over time is integratedwithin a time frame of roughly 200 ms and, in echo perception, both echoduration and intensity is factored. Accordingly, for accurate echocanceling performance testing, it would be desired to calculate both theecho spike energy and the energy spike duration and integrate thesevalues into one performance number.

The method, according to the present invention, utilizes an associatedalgorithm that examines the convergence period energy escaping from anecho canceller's S_(out) port and calculates a single statistic torepresent this echo (Erep). Briefly, the new algorithm analyzes theresidual echo energy (both power and duration) observed duringperformance testing and represents this echo energy in a single scorethat can be used to represent how well the echo canceller will performin the presence of speech. Pass/fail criterion can then be developedbased on correlation studies (measured echo to mean opinion score (MOS)results based on testing with multiple speech samples) that can serve asa new and more accurate requirement on echo canceller convergence speed.

For purposes of the present invention, an echo spike is defined as aburst of energy having a duration in excess of 5 ms and a level greaterthan −50 dBm. The −50 dBm threshold is based on much evidence that briefecho at and below −50 dBm has little negative subjective effects. Thus,one skilled in the art will recognize that this threshold is chosenempirically so as to maximize the predictive validity of the algorithmand that other thresholds may be used.

For each echo spike that exceeds the 5 ms/−50 dBm threshold, thealgorithm multiplies the duration and energy to get a single score.Then, the algorithm sums the scores of such echo spikes to obtain anoverall test score. The algorithm is expressed in the followingequation:

Erep=aΣt _(i) p _(i); t_(i)>=5 ms;

where “t_(i)” is the ith echo spike duration (ms), “p_(i)” is the ithecho spike's average power level (dBm), “a” is an arbitrary scalingfactor and “Erep” is the final score for the echo signal file. (Asuitable scaling factor “a” for purposes of the present invention is0.01.) Thus, the method according to the present invention includes thesteps of calculating the power and the duration of each spike within aresidual echo energy and then integrating the calculations over all thespikes seen to yield a single objective score. This algorithm may beadvantageously implemented on programmable test equipment using aprogrammable computer, discrete digital circuits or application specificintegrated circuits (ASICs).

Verification tests of the present invention show that the objectivescore achieved with the new test signal and algorithm is highlycorrelated (0.89) with the MOS results. Thus, the test result achievedwith the method of the present invention is highly predictive of howwell an EC will perform when acting on actual speech signals.

Example

To evaluate the new 4-band test signal and method according to thepresent invention, the following objective tests and comparisons to MOStest results were conducted.

Four different echo cancellers (ECs) were tested using the test signaland method of the present invention: EC1; EC2; EC3; and EC4. Inaddition, three different types of objective test signals were used: 1)the 4-band test signal of the present invention; 2) the CSS and theG.165 white noise signal; and 3) 8 different speech signals, eachrepresenting a different speaker (4 female, 4 male) speaking a uniquepair of short sentences. Each test signal was processed through each ECat 5 different echo return loss (ERL) levels: 6, 8, 10, 12 and 14 dB.During this processing, the energy appearing at the EC's S_(out) port(i.e., the echo energy not captured by the EC) was recorded. Theplay/record process was done by a computer system equipped with aspecial dual-T1 board. Any computer equipped with the necessary hardwareinterface and software can be used to accomplish the play/record. Thecomputer was connected to the test EC, either directly or through a PBX.The recorded echo samples were each processed via the method accordingto the present invention to generate an Erep score.

To prepare the speech-based recordings for use in the subsequent MOStest, 180 ms of “delay” was added to the front of the recorded echosamples obtained when the 8 speech signals were in use to simulate theround trip delay of a digital cellular connection and then these weremixed with the source samples. Where significant echo energy is presentin the recordings, the delay and mixing strategy colors the sourcesamples with the echo. These mixed files were subsequently rated forquality (by 33 subjects) within an MOS test.

The criterion chosen for best objective signal is the one whosecalculated Erep best correlates with the MOS results obtained inresponse to the speech samples that were processed. To be a generallyuseful test signal the absolute correlation obtained needs to be high.

FIG. 4 shows the MOS result for each of the four ECs at each of the 5ERL levels. The error bars are standard deviation. From the graph it canbe seen that ECs 1, 2 and 4 exhibit the expected positive relationshipbetween MOS and ERL. For EC3, the MOS is flat across the ERL range. Thedifference among ECs is likely due to the complex result of differentadaptive filter designs working in conjunction with different NLPthresholds. That said, it is expected that all ECs will show fulloperation (convergence) at and above some ERL level.

FIG. 5 plots the relationship between the Erep calculated on therecordings made with the speech signals and the MOS results across thefour ECs. Each data point is the mean of 8 speaker files. The obtainedcorrelation of −0.97 is very high, although it is somewhat expectedsince both the calculated Erep and MOS data are based on the samerecordings.

FIG. 6 plots the Erep for the 4-band signal as a function of both EC andERL. The general shape is in good agreement with the MOS results in FIG.4, i.e., the Erep for EC3 is little affected by the ERL manipulationwhereas a greater ERL effect is seen for the other 3 ECs in the sample.

FIGS. 7 a, 7 b and 7 c plot MOS (collapsed over the EC variable) againstErep for each of the 4-band, white noise and CSS signals. The 4-bandsignal obviously correlates best with the MOS score. It can be seen thatthe MOS score stays above 4 when the Erep is below 100. That suggestsfor the current 4-band signal level that if the score is below 100 theecho is not noticeable. The data presented in FIG. 5, based on the Erepfor speech signals, would lead to a similar conclusion for the testsignal. The overall data suggested that a pass/fail criterion ofErep<100 might be too stringent. Thus, taking the ERL variable intoconsideration, the criterion can be relaxed slightly whereby Erep<200for ERLs>10 can be adopted, since ERLs below 10 dB are rarelyencountered in the field.

Another relationship of interest is that between the 4-band signal'sErep and the speech sample's Erep. As shown in FIG. 8, the 4-band signalresults correlate very high with the speech sample results. This lendsfurther support for the utility of the 4-band signal of the presentinvention.

As a result of the present invention, a better objective test signal andan associated measurement method are provided. By using a more accuratetest signal, a network service provider will be able to reduce its costsof conducting pre-deployment echo canceller testing. Although the focushere was on echo canceller convergence speed, the 4-band test signal andmeasurement algorithm should be useful in testing other echo cancellerperformance areas since the underlying factor, residual echo, isconstant.

While there has been described what is presently believed to be thepreferred embodiments of the invention, those skilled in the art willrealize that various changes and modifications may be made to theinvention without departing from the spirit of the invention, and it isintended to claim all such changes and modifications as fall within thescope of the invention.

1. Test equipment to test an echo canceller, the test equipmentcomprising a signal generator to generate a white noise test signalassociated with an overall frequency band, the white noise test signalincluding a plurality of high-level white noise time segments each ofwhich is separated by a low-level white noise time gap, the high-levelwhite noise time segments comprising: a first segment associated with afirst frequency band of the overall frequency band; a second segmentfollowing the first segment and associated with a second frequency bandof the overall frequency band, the second frequency band being higherthan the first frequency band; a third segment following the secondsegment and associated with a third frequency band of the overallfrequency band, the third frequency band being higher than the secondfrequency band; and a fourth segment following the third segment andassociated with a fourth frequency band of the overall frequency band,the fourth frequency band being higher than the third frequency band. 2.The test equipment as defined in claim 1, wherein the white noise testsignal has an overall frequency band from about 0 kHz to about 3.5 kHz.3. The test equipment as defined in claim 1, wherein: the firstfrequency band of the first segment is from about 0 kHz to about 1 kHz;the second frequency band of the second segment is from about 1 kHz toabout 2 kHz; the third frequency band of third segment is from about 2kHz to about 3 kHz; and the fourth frequency band of the fourth segmentis from about 3 kHz to about 3.5 kHz.
 4. The test equipment as definedin claim 1, wherein the white noise test signal has a period of about3900 msec.
 5. The test equipment as defined in claim 1, wherein eachhigh-level white noise time segment of the plurality of high-level whitenoise time segments has a level of about −18 dBm.
 6. The test equipmentas defined in claim 1, wherein each high-level white noise time segmentof the plurality of high-level white noise time segments has a durationof about 350 msec.
 7. The test equipment as defined in claim 1, whereinthe low-level white noise time gap has a level of about −65 dBm.
 8. Thetest equipment as defined in claim 1, wherein the low-level white noisegap has a duration of about 150 msec.
 9. A method of testing an echocanceller, the method comprising: generating a white noise test signalthat includes a plurality of high-level white noise time segments eachof which is separated by a low-level white noise time gap, thehigh-level white noise time segments including at least a first segmentassociated with a first frequency band, a second segment following thefirst segment and associated with a second frequency band that is higherthan the first frequency band, a third segment following the secondsegment and associated with a third frequency band that is higher thanthe second frequency band, and a fourth segment following the thirdsegment and associated with a fourth frequency band that is higher thanthe third frequency band; transmitting the generated white noise testsignal to the echo canceller; receiving at least one echo output signalfrom the echo canceller, the echo output signal resulting from the whitenoise test signal; and measuring level and duration of the received atleast one echo output signal.
 10. The method as defined in claim 9,further comprising: calculating a performance score based on themeasured level and duration of the at least one echo output signal. 11.The method as defined in claim 9, wherein the echo output signal has aduration greater than about 5 msec and a level greater than about −50dBm.
 12. The method as defined in claim 10, wherein calculating theperformance score comprises: multiplying the measured level and durationof each output echo signal of the at least one echo output signal toobtain a product; and summing the product of each output echo signal ofthe at least one echo output signal.
 13. The method as defined in claim9, wherein: the first frequency band of the first segment is from about0 kHz to about 1 kHz; the second frequency band of the second segment isfrom about 1 kHz to about 2 kHz; the third frequency band of the thirdsegment is from about 2 kHz to about 3 kHz; and the fourth frequencyband of the fourth segment is from about 3 kHz to about 3.5 kHz.
 14. Themethod as defined in claim 9, wherein each high-level white noise timesegment of the plurality of high-level white noise time segments has alevel of about −18 dBm.
 15. The method as defined in claim 9, whereineach high-level white noise time segment of the plurality of high-levelwhite noise time segments has a duration of about 350 msec.
 16. Themethod as defined in claim 9, wherein the low-level white noise time gaphas a duration of about 150 msec.
 17. The method as defined in claim 9,wherein the low-level white noise time gap has a level of about −65 dBm.18. A computer-readable storage medium comprising operationalinstructions that, when executed by a processing device, cause theprocessing device to: generate a white noise test signal that includes aplurality of high-level white noise time segments each of which isseparated by a low-level white noise time gap, the high-level whitenoise time segments including at least a first segment associated with afirst frequency band, a second segment following the first segment andassociated with a second frequency band that is higher than the firstfrequency band, a third segment following the second segment andassociated with a third frequency band that is higher than the secondfrequency band, and a fourth segment following the third segment andassociated with a fourth frequency band that is higher than the thirdfrequency band; transmit the generated white noise test signal to theecho canceller; receive at least one echo output signal from the echocanceller, the at least one echo output signal resulting from the whitenoise test signal; and measure level and duration of the received atleast one echo output signal.
 19. The computer-readable storage mediumas defined in claim 18, further comprising operational instructionsthat, when executed by the processing device, cause the processingdevice to: calculate a performance score based on the measured level andduration of the at least one echo output signal.
 20. Thecomputer-readable storage medium as defined in claim 18, wherein the atleast one echo output signal has a duration greater than about 5 msecand a level greater than about −50 dBm.
 21. The computer-readablestorage medium as defined in claim 19, wherein instructions that causethe processing device to calculate the performance score compriseinstructions that, when executed by the processing device, cause theprocessing device to: multiply the measured level and duration of eachoutput echo signal of the at least one echo output signal to obtain aproduct; and sum the product of each output echo signal of the at leastone echo output signal.
 22. The computer-readable storage medium asdefined in claim 18, wherein: the first frequency band of the firstsegment is from about 0 kHz to about 1 kHz; the second frequency band ofthe second segment is from about 1 kHz to about 2 kHz; the thirdfrequency band of the third segment is from about 2 kHz to about 3 kHz;and the fourth frequency band of the fourth segment is from about 3 kHzto about 3.5 kHz.
 23. The computer-readable storage medium as defined inclaim 18, wherein each high-level white noise time segment of theplurality of high-level white noise time segments has a level of about−18 dBm.
 24. The computer-readable storage medium as defined in claim18, wherein each high-level white noise time segment of the plurality ofhigh-level white noise time segments has a duration of about 350 msec.25. The computer-readable storage medium as defined in claim 18, whereinthe low-level white noise time gap has a duration of about 150 msec. 26.The computer-readable storage medium as defined in claim 18, wherein thelow-level white noise time gap has a level of about −65.